The invention relates to a method of and an apparatus for controlling the frequency of at least two auxiliary signal frequencies with constant phase difference therebetween for the demodulation of a phase-keyed high-frequency signal.
In the transmission of information by the phase keying of a high-frequency carrier, the phase of the carrier is altered at equal time intervals by integral multiples of a definite phase angle corresponding to the data signals to be transmitted. For demodulation at the receiver of the transmitted high frequency signal or of a signal of lower frequency derived therefrom by frequency conversion, signals are derived which correspond at all times to the mean product of the transmitted signal and at least one auxiliary signal of constant frequency. In this synchronous demodulation, the frequency of the auxiliary signal must, by automatic regulation, be held continuously in agreement with the frequency of the signal to be demodulated when in the unchanged keying condition. The control voltage necessary for frequency control of the auxiliary signal must however be generated even with changing keying and it should be independent of the instantaneous keying condition.
Such a regulating voltage may be derived by frequency multiplication of the signal to be demodulated and an auxiliary signal and formation of the mean product of the two signals of higher frequency. Alternatively, a frequency multiplication and subsequent frequency division of the signal to be demodulated is also possible to obtain a carrier with suppressed phase modulation, of which the phase is then compared with the phase of an auxiliary signal to produce a control voltage. A considerable disadvantage of such methods results from the additional expense of this multiplication and the filtering made necessary by the use of this technique. With fast keying the transient delays of the filters employed may also operate deleteriously.